1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device with memory cells including ferroelectric capacitors.
2. Description of the Related Art
Semiconductor memory devices have been used in a variety of electronic apparatuses, because of such attractive features as small size and large memory capacity. In particular the dynamic RAM is more frequently used than other types of semiconductor memory devices such as the static RAM, because the dynamic RAM may realize a large memory capacity with a small chip area.
A memory cell of the typical dynamic RAM consists of a capacitor for storing data and a MOS transistor for charge transfer. Data is stored in the memory cell by charging the capacitor. A charged state of the capacitor is assigned to storage of data "1", and a noncharged state of the capacitor, to storage of data "0".
As is well known, an amount of charge stored in the capacitor gradually decreases with time due to its leakage. The charge amount decrease phenomenon inevitably occurs also in the memory cells of the dynamic RAM, leading to the deterioration of data stored therein. To cope with this problem, the memory cells are periodically refreshed to recharge the capacitor. The refreshing operation is not performed when the power source is in an off state. Accordingly, when the power switch is turned off, the data in the dynamic RAM is erased.
Thus, the dynamic RAM is advantageous in terms of its small size and large memory capacity, but requires a refreshing operation and hence complicated control.
Recently, many studies have been made to develop a new type of memory device which has a simple cell structure like the dynamic RAM but, unlike the dynamic RAM, does not require a refreshing operation.
Such a new type of memory device, called FRAM (ferroelectric RAM), is described in "Electronics", Feb. 18, 1988, pp 91 to 95 and "IEEE JOURNAL OF SOLID-STATE CIRCUIT", Vol. 23, No. 5, October, 1988, pp 1171 to 1175. The FRAM uses ferroelectric capacitors for data storage. The ferroelectric capacitor has a property that, when it is placed under a predetermined applied voltage, a predetermined electric field is developed therein, and the capacitor retains the electric field even after the applied voltage is removed.
Thus, the FRAM's memory cells do not require refreshing, due to its electric field retaining property.
To secure bi-stable storage states, data "1" and "0", the FRAM needs an additional circuit function to reverse the polarity of the field retained in the ferroelectric capacitor. The above memory device realizes this function by applying a pulse signal to one of the electrode plates of the ferroelectric capacitor. This approach is indeed advantageous in eliminating the refreshing operation. However, it needs an additional circuit for generating pulses, making the entire memory device circuit complicated.